This disclosure relates to methods for manufacturing a filling in a gap region between two surfaces, for example, an underfill for flip-chip packages.
In modern electronic devices, substantial gains in performance are continuously achieved by means of circuit miniaturization and by the integration of single package, multi-functional chips. The scalability and performance of such electronic devices are related to their ability to dissipate heat. In typical flip chip arrangements, one integrated circuit (IC) surface is used for heat removal through a heat sink, while the other for power delivery and data communication. Power is delivered throughout solder balls attached to electrical pads on the IC chip that are flowed and coupled to the main circuit board.
To minimize mechanical stress in the solder balls and to protect them electrically, mechanically, and chemically, the gap region between the IC chip and board (created due to the presence of solder balls) is conventionally filled with electrically non-conductive materials, known as underfills. Current efforts towards 3D chip integration, with solder balls as electrical connection between silicon dies, demand high thermally conductive underfills to efficiently dissipate the heat of lower dies to the heat removal embodiment attached at the chip stack backside.
Conventional underfills consist of a curable matrix (e.g., epoxy resin) loaded with silica fillers, which have a similar thermal expansion coefficient (CTE) to that of the silicon. Currently, the requirement of matching CTE dictates the type, and volumetric fill of fillers to be employed in a given underfill. For thermal underfills the thermal conductivity of filler materials which are used to increase the thermal contact and enhance heat dissipation between connected surfaces should be high. Therefore, Al2O3, AlN, BN or other metal and nonmetal materials, for example are used.
The application of underfills in gap regions is limited by the filler volume fraction, since the resulting viscosity depends on the filler content. According to some conventional methods the underfill material is applied to the chip periphery and capillary forces transport the viscous media into the gap, within a certain time period, prior to a temperature assisted curing. Generally, a high particle load, e.g., >30 vol % is needed to reach thermal conductivity values >0.5 W/m/K. Then, the viscosity of the applied medium may become too high to efficiently fill the gaps. Therefore, vacuum—or pressure-assisted filling processes, for example, as disclosed in U.S. Pat. No. 6,000,924 were proposed. However, the resulting thermal performance of the underfill may not be sufficient as it is required for 3D-integrated chips.